This invention relates to a variable stage type carburetor and particularly to a variable stage type carburetor which can prevent a lean mixture charge at the time when the associated engine is operated with heavy load.
A variable stage type carburetor contemplates that the opening area of a venturi portion defined between a movable vane and the inner wall of an intake passage is varied by the movable vane operatively connected to a throttle valve via a link mechanism so that the venturi vacuum is controlled, while a fuel metering area is varied through cooperation of a jet needle connected to the movable vane with an orifice provided in the venturi wall surface confronting the vane so that an air-fuel ratio is controlled to a given value. This type of carburetor can set the venturi vacuum at a considerably high value and has an excellent fuel atomizing characteristics.
In the conventional variable stage type carburetors, however, the venturi vacuum is inconveniently lowered as the intake air flow rate is reduced, because the opening degree of the movable vane is held corresponding to the opening degree of the throttle valve, irrespective of the intake air flow rate. Thus, when a heavy load is applied to the engine associated with this type of carburetor, the revolution speed of the engine shaft is lowered to decrease the intake air flow rate, so that the venturi vacuum is lowered. Consequently, the amount of fuel induced into the intake air flow is reduced to make the air-fuel mixture lean, resulting in a lowered engine output.
In this connection, the specification of U.S. Pat. No. 4,005,161 (Shoji Shimo et al) discloses an improved carburetor of the variable stage type. This carburetor has a vane controller adapted to move the movable vane, the opening degree of which is changed in direct proportion to the opening degree of the throttle valve, further toward the closing position, in response to the lowering of the venturi vacuum. In this improved carburetor, however, the vane controller is designed such that it is turned into effect only when the opening degree of the throttle valve exceeds a predetermined opening degree, since it is extremely difficult to obtain a good functioning of the vane controller over entire range of opening degree of the throttle valve.
Thus, the vane controller is not effective when the throttle valve has not been opened beyond the predetermined opening degree. Therefore, when the intake air flow rate is decreased during the engine operation with the throttle valve opening degree not exceeding the predetermined opening degree, the venturi vacuum is excessively lowered to render the mixture extremely lean, resulting in a deteriorated or unstable engine operation and, in the worst case, stalling of the engine. At the same time, a malfunctioning of the engine may be caused by an extraordinary drop of venturi vacuum attributable to a lowering of the intake air flow rate beyond the range of the effectiveness of the vane controller.
The U.S. Patent Ser. No. 828,670 (K. Karino et al) filed on Aug. 29, 1977, U.S. Pat. 4,105,720 is concerned with a carburetor having a vane controller and an acceleration pump in combination. Although the acceleration pump can compensate for the decrease of the fuel supply rate to some extent, when the intake air flow rate is extraordinarily lowered during the engine operation with small throttle valve opening degree, this carburetor is still unable to provide an ultimate satisfaction.